Harding's Luck by Edith Nesbit

Harding's luck is sequel to E. Nesbit's "The House of Arden". It tells the story of Dickie Harding, a disabled boy, who one day accidentelly discovers an old magic, that allows him to travel into his own past. There he meets Elfrida and Edred Arden (as told in "The House of Arden") and together they seek for a long lost treasure.